The objectives of the proposed study are: (1) to establish the relative importance of vasomotion upon the energy balance of chickens and (2) to investigate the role of the central nervous system upon the partitioning of blood through the capillaries or the arteriovenous anastomoses (AVAs). The importance of vasomotion will be determined by direct measurements of convective and radiant heat loss using partitional gradient layer calorimetry. Two "organ" calorimeters have been constructed to measure heat loss from the two major sites of known vasomotr control--feet and head. Heat loss from the feathered surfaces will be measured by the existing calorimeter. Spontaneous vasomotor control and induced vasomotor control will be monitored below, at and above thermonuetrality of unanesthetized chickens. Vasoconstriction reduces blood flow in the foot to a low of 0.23 ml/min., while vasodilation increases flow to as much as 4.7 m./min. Using microspheres to measure the relative contribution of AVAs to blood flow in the foot, we measured an AVA flow of 0.006 ml/min. during vasoconstriction and 0.17 ml/min. during vasodilation. Flow through capillaries appears to be under both alpha and beta - adrenergic control, while AVAs appear to be predominately under beta - adrenergic control. Vasodilation of both capillaries and AVAs appears to occur from the release of vasomotor tone.